1. Field of the Invention
This invention relates to an automatic focus controlling device for an optical instrument such as a photographic camera or an endoscope, and more particularly to a device for automatically detecting the distance of an object and controlling focus based on the detected distance which is adaptable to any existing optical instrument.
2. Description of the Prior Art
In an optical instrument having an objective such as a photographic camera or an endoscope, it is desirable to automatically control the focus of the image of the object to be viewed or photographed. There have been developed and suggested various kinds of focus detecting or adjusting devices for photographic cameras. They are, however, provided with a special and complex focus detecting means in addition to the conventional structure of the optical instrument.
Particularly in an endoscope, it is undesirable to provide an additional focus detecting device at the head of the flexible optical fiber tube thereof because the head is desired to be as thin as possible to minimize the pain to the patient.
In an endoscope, it has been known to project an image of a pinhole onto the object to be observed and detect the image on the object to adjust the focus as disclosed in Japanese Unexamined Patent Publication No. 12193/1978 filed July 20, 1976. In this focus adjusting system using a pinhole image, the focus adjustment must be conducted before the photographing operation and accordingly the projection of the pinhole is turned off when photographing. Further, the fiber tube is partly used for projecting the pinhole image and not fully used for viewing the object. Thus, the structure and the operation of the endoscope are complex due to the additional optical system and operation required for projecting and viewing the pinhole image and also adjusting focus.
Further, it has been known in the art to use a spatial frequency filter to detect the focus. However, since the contrast of the image within the body cavities is very low, the accuracy of the focus detection by use of the spatial frequency filter cannot be made so high.